Conventionally, the manufacture of semiconductor devices such as LSI and ultra LSI devices, or liquid crystal display panels and the like, involves patterning of semiconductor wafers or liquid crystal base plates through irradiation of light upon them.
However, a problem exists in that any dust particles adhering to the exposure original plate (photomask) used in such cases absorb and reflect light, which deforms and roughens the edge lines of the transferred patterning, thereby impairing the dimensions, quality, appearance and performance of the semiconductor device and/or liquid crystal display panel, while reducing the manufacturing yield thereof.
Thus, these procedures are usually carried out in a clean room; but keeping the photomask always in good conditions even within such a clean environment is difficult, and hence a pellicle with a membrane having good transmittance to the exposure lights is mounted on the surface of the photomask, for dust-proof protection. Doing so is advantageous in that the dust particles are not deposited directly onto the surface of the photomask, but become deposited onto the pellicle membrane, so that during photolithography the dust particles on the pellicle membrane do not affect image transfer, since the photo focus is set on the pattern formed on the photomask.
A pellicle is built up of a pellicle frame, which is made of aluminum or a stainless steel or polyethylene or the like, and a transparent pellicle membrane made of cellulose nitrate or cellulose acetate or the like, which transmits light well; this membrane is attached to one of the two frame faces (hereinafter referred to as “membrane-receiving frame face” or “membrane-bearing frame face”) after laying a solvent capable of dissolving the pellicle membrane on the membrane-receiving frame face and drying the solvent by air flow (ref. Publication-in-patent 1), or after laying an adhesive such as acrylic resin and epoxy resin (ref. Publications-in-patent 2, 3 and 4); furthermore, on the other one of the two frame faces (hereinafter referred to as “mask-side frame face”) is laid an adhesive layer made of a polybutene resin, a polyvinyl acetate resin, an acrylic resin, a silicone resin or the like, and over this adhesive layer (hereinafter also referred to as “agglutinant layer” to distinguish it from the adhesive layer for bonding the membrane, though there may be cases where these two layers are made of an identical adhesive material) is laid a releasable liner (separator) for protecting the agglutinant layer.
In recent years, the requirement for the resolution of lithography has become heightened gradually, and in order to attain such higher resolutions the light sources having shorter and shorter wavelengths have come to be adopted. In practice, ultraviolet lights [g-line (436 nm), I-line (365 nm), KrF excimer lasers (248 nm)] are newly employed, and more recently ArF excimer lasers (193 nm) have begun to be used.
As the wavelengths of the exposure lights are shifted toward shorter lengths, a new problem has arisen wherein a deformation of the lithographic image is caused by the deformation of the exposure original plate (mask).
It has been pointed out that one of the causes for the deformation or in-plane distortion of the exposure original plate is the less admirable flatness of the pellicle which is attached to the exposure original plate. The inventor hereof previously presented a proposal for controlling the pellicle-induced distortions of mask by means of an improvement in the flatness of the mask-bonding adhesive layer, i.e., the agglutinant layer (ref. Publication-in-patent 5).
In this Publication-in-patent 5, it is proposed to make flatter the surface of the agglutinant layer laid on the mask-side frame face of the pellicle frame by pressing the pellicle frame on a flat plate having a high flatness by the weight of the pellicle frame itself.
That invention certainly improved the maintenance of the high flatness of the mask greatly; however, there have still been occasional incidents observed wherein the transferred light image was deformed, especially in the cases wherein the masks are exposed to lights of shorter wavelengths. The cause for this deformation was looked for and it was found that the flatness level of the membrane-side surface of the pellicle, which had been thought irrelevant, had a subtle relevancy.
Thus, when the pellicle is being attached to the mask, the membrane-side surface of the pellicle is touched by a pressure plate of a pellicle mounter, and on this occasion, if the membrane-bonding adhesive layer has an unevenness in its surface, the convex parts are pressed with greater forces than concaved parts are. If the (mask-side) surface of the agglutinant layer is finished sufficiently flat, the pellicle frame itself does not suffer any substantial deformation even at the thicker localities of the membrane-bonding adhesive layer, thanks to the relatively high rigidity of the pellicle frame as a whole, but the pressure that comes by way of the pellicle frame either causes the side faces of the agglutinant layer to bulge and thereby get in contact with the mask, or causes more compressive residual stress to build up at such localities of the agglutinant layer.
When the pressure plate is removed from the pellicle and the imposition of the pressure is let up, the agglutinant layer freed from the pressure starts trying to regain its former shape. Owing to the sizable adhesive strength and resiliency of the adhesive constituting the agglutinant layer, that portions of the agglutinant layer which are bulging from the side faces of the agglutinant layer are liable to remain affixed to the mask surface wherefore the agglutinant layer as it recovers its former shape pulls the mask surface, or those portions of the agglutinant layer where more compressive residual stress is built up are inclined to expand the mask, wherefore the in-plane distortions of mask occur as the pellicle is bonded on the mask.